Piston-type variable attenuator



2 Sheets-Sheet l Filed Dec. 27, 1950 aff.:

INVENTOR.

ROGER J. PIERCE BY 7W Feb 8, 1955 R. J. PIERCE 2,701,863

PISTON-TYPE VARIABLE ATTENUATOR Filed Dec. 27. 1950 2 Sheets-Sheet 2 IN VEN TOR.

ROGER J. PIERCE nited States Patent() PISTON-TYPE VARIABLE ATTENUATOR Roger J. Pierce, Honolulu, Territory of Hawaii, assignor to the United States of America as represented by the Secretary of the Army Application December 27, 1950, Serial No. 202,933

4 Claims. (c1. sas- 81) This invention relates to improvements in piston-type variable attenuators and more particularly to those wherein two co-planar loops or discs provide mutualinductance coupling between an input coaxial transmission line and an output coaxial transmission line, the action being such that, as the distance between the two loops or discs is varied, the coupling, and therefore the attenuation, between input and output circuits is varied. The principle of mutual-inductance piston-type attenuators is well known and involves, basically, the action wherein the energy introduced at the input loop or disc attenuates exponentially along the tube formed by the outer conductor, to be transferred to the output loop or disc circuit. For a more complete discourse on this principle, reference may be made to the paper by E. G. Linder, entitled Attenuation of Electromagnetic Fields in Pipes Smaller Than the Critical Size and published in the Proceedings of the Institute of Radio Engineers, vol. 30, No. 12, December 1942; and to the paper by David E. Harnett and Nelson P. Case, entitled` The Design and Testing of Multirange Receivers and published in the Proceedings of the Institute of Radio Engineers, vol. 23, No. 6, June 1935.

An example of a variable attenuator of the general type to which the present invention relates, is shown and described in Patent No. 2,427,643 issued September 16, 1947 to John Collard. Reference may also be made to Patent No. 2,468,688 issued April 26, 1949 to Otto H. Schmitt; to Patent No. 2,233,166 issued February 25, 1941, to William C. Hahn; to pages 707 to 709 and 770 to 774 in vol. l1 of Radiation Laboratory Series; and to pages 1016 to 1018 in vol. II of Very High-Frequency Techniques. The latter are 1947 publications of the McGraw-Hill Book Company, Inc., New AYork and London.

In the various constructions of piston-type variable attenuators proposed heretofore, input and output connections move vwith respect to each other. It has therefore been necessary, in these prior designs, that at least one of the connections be made from a flexible cable to permit movement of the input and output connections relatively with respect to each other.

Among the objects of the present invention is the provision of an improved piston-type variable attenuator wherein the physical length between input and output connections is held or remains constant as attenuation is varied. v

Other objects and advantages will hereinafter appear.

In accordance with the invention, a constant-length radio-frequency piston-type attenuator of the mutualinductance variety is provided with a movable piston part or coaxial section which is slidable telescopically between a fixed input section and a fixed output section thereby to effect the desired variation of attenuation without necessitating a change in the physical length between input and output connections, as heretofore.

For the purpose of illustrating the invention, an embodiment thereof is shown in the drawings, wherein Figs. 1 and la are enlarged, longitudinal, sectional views which, when fitted together at x--x, show a radiofrequency variable attenuator constructed and having operating characteristics in accordance with the present invention, the section in each being taken on the line 1 1 in Fig. 2 and some of the parts being removed for the sake of simplicity; and

Figs. 2 and 3 are sections taken, respectively, on the lines 2-2 and 3-3 in Fig. 1.

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With reference to Figs. 1 and la, input to the improved attenuator is fed to a fixed input coaxial section the inner conductor 10 of which terminates in a fixed coupling element or input loop 11. The latter is grounded to the outer cylindrical conductor 10a of the input section through a stud 12 screwed into the end 14 of a cylindrical housing 15. Conductor 10 is threaded to receive nuts 10b disposed on opposite sides of a supporting disc 14 which is secured to the housing end 14, as shown in Fig. 1.

The fixed output coaxial section of the attenuator consists of a tubular inner conductor 16, a tubular outer conductor 16a, and a polystyrene spacer 16b interposed between the two. Spacers 17 and 18 disposed as shown between housing 15 and the outer conductors 10a and 16a, respectively, are soldered or otherwise secured to these parts.

The important structural feature of the improved at-y tenuator to which is attributed its better operating characteristics over the various designs proposed heretofore, resides in the movable intermediate coaxial section or piston part which is telescopically disposed with respect to the rigidly fixed input and output coaxial sections and is slidable between the latter, along the axis of the attenuator. This movable coaxial section or piston part consists of complementary tubes 19 and 20 disposed one within the other, and an interposed spacer ring 21 soldered or otherwise secured to these tubes, as indicated in Fig. 1. Tube 19 is disposed Within and has a relatively free sliding fit with respect to the tubular housing 15. Tube 20 which serves as the outer conductor of the movable coaxial section is disposed within the outer conductor 10a of the fixed input coaxial section but fits over the outer conductor 16a of the fixed output coaxial section, and has a relatively free sliding fit with respect to both of the conductors 10a and 16a. Brass rings 22 and 23 and a polystyrene spacer 24 between the same, have a press t in tube 20 and are disposed close to the free end 20a thereof which may be cut or slit longitudinally to provide eight to ten fingers (not shown). The latter are given sufficient spring radially outwardly to make good sliding, electrical contact with the adjacent surface` of conductor 10a. The free end 10c of conductor 10a may be similarly cut or slit longitudinally to provide spring fingers (not shown) which press radially inwardly to make good sliding electrical contact with the adjacent surface of tube 20 serving as the outer conductor of the movable coaxial section.

The inner conductor of the movable coaxial section is a brass rod 25 one end of which is threaded and passes through spacer 24 and terminates in a movable coupling element or output loop 26. The latter is grounded to the outer cylindrical conductor 20 of the movable coaxial section through a stud 27 screwed into ring 23. The

other end of rod 25 is received by and has a relativelyl extending slot 15a which accommodates a rack 27 one end of which is secured to the movable coaxial section by a ilathead screw 28 threaded into spacer 21. The other end of rack 27 is bolted at 28 to the tube 19. Fixed on a shaft 29 is a pinion 30 in mesh with rack 27.

The rack-and-pinion unit shown is of a conventional design, and this or any other suitable means may be employed to slide or adjust the movable coaxial section axially of the attenuator whereby the movable coupling element 26 is moved toward or away from the fixed coupling element 10, thereby to effect variation in attenuation. In the specific construction shown, this adjustment or variation is made by manual rotation of a knob 31 fixed on shaft 29. Mounted on shaft 29 adjacent to knob 31 is a dial 32. The spacing of teeth on the rack, the diameter of pinion 30, and the dial engraving may be so chosen that one dial division corresponds to one decibel of attenuation, for example. Energy introduced at the input loop 11 attenuates exponentially along the tube formed by the outer conductor to be trans.- ferred to the output loop circuit.

'Discs Aor other suitable means may be used for the xed Yand `movable A'coupling elements vin lieu of vthe iloops 11 and 26.

From the foregoing it willbe seen that in the improved piston-type variable attenuator shownanddescribedherein, the physical :length :between 1input :and output iconnections ,is held :constant as :attenuation riswaried, wherefore the :necessity :of using iflexible connecting cables is avoided.

While but one embodiment ofthe invention has :been shown and described, :it will .-be understood lby those skit-led in the art `.that various changes :such as `in `,the size, rshape, and 'arrangement of .the parts, kifnay :be made ywithout `departing vfrom the :spirit of the invention or the .scope i ther-claims.

Ihe invention claimed is:

1. :In .a piston-type variable attenuator .of the character described, input .and output connections `rigidly fixed with ,respect -to eachother, .a tubular iirst .coaxial section rigidly xed with respect to one of said connections Iand comprising :an outer .and inner conductor, a second coaxial section rigidly ixed with :respect to the other .of said .connections and comprising ahollow linner conductor ,and :an outer fconductor whose .outer ,diameter issmaller lthan the innerdiameter `of the .outer conductor of said first coaxial section, a tubular third coaxial section comprising a tubular :outer conductor whose .outer diameter .is smaller `than the diameter of said irstsection outer `conductor vand Vwhose inner diameter :is Ilarger *than the ,outer diameter of the ysaid second section .outer conductor, the outer conductor of said intermediate section having a sliding lit with respect to the outer conductors of .both of the said irst and second `coaxialsections, said intermediate section having an inner conductor telescoped and having a sliding tiwithin said innerconductor of said second coaxial section, a rst coupling Velement associated with said iirst coaxial section, a -second coupling element `associated with said third ycoaxial section and arranged `to provide with said iirst coupling element a mutual-inductance 'coupling :between :said irst and ysecond coaxial sections, and means for imparting .an axial.slid ing .movement to said third coaxial section.

2. A piston-type variable attenuator comprising, .a slotted housing, Va coaxial input Vsection -enclosed within and rigidly aixed to one end of said housing and .comprising `outer and inner conductors, a liirst coupling loop attached to one end of said inner conductor and enclosed within said outer conductor, a coaxial output section enclosed within and vrigidly iixed to the `other end yof -said housing and .comprising #outer and :hollow ,inner conductors, a coaxial intermediate section enclosed within said housing between said input and output sections and comprising an outer conductor sandwiched between and in slidable fcontact with the outer yconductors Yof said .input and output sections, the dimensions of the outer conductors of .said finput, output and intermediate -sections being such that the louter conductor of said intermediate section .is slidable over the outer .conductor'of said zoutput section and Ainside the outer conductor .ofsaid 'input section, one v,end of the inner conductor rof .said :intermediate section being in contact with and slidable within the hollow inner conductor of said output section and its ,other end being attached to one end of the outerconductor of said intermediate section, a :second coupling loop attached to said other end of said intermediate section inner .con-

.4 ductor and disposed within the outer conductor of said intermediate section, ysaid 'first yand 'second loops providing a mutual inductance coupling between said input and said output coaxial sections, a rack that is slidable in the slot of said housing and attached to the other end of said other intermediate section outer conductor, said rack being parallel to the longitudinal laxis of said intermediate section outer .conductor .and extending substantially the full length thereof, a pinion aixed to said housing and in mesh with said rack for imparting a longitudinal movement 'to said intermediate section .to cause a change .in the mutual inductance coupling between `said input .and said output sections without changing the overall dimensionsfof 'said attenuator.

3. In a piston-type variable attenuator of the character described, input and outputconnections rigidly iixed with respect to each other, input and output tubular coaxial sections respectively lixed to said input and output connections, an lintermediate Vtubular coaxial vsection common to said .input and output coaxial sections and disposed :and dimensioned for 'sliding inside one of said .first named vcoaxial sectionsand for simultaneous sliding movement :outside of the 4other .of said iirst -named coaxial sections, -a first electromagnetic energy Icoupling .element lying `within .and -in fixed relationship with :respect tto` `one :of v.said rst named :coaxial sections, .and .a second .electromagnetic energy coupling element :lying within and in :fixed relationship .with respect :to said .intermediate :coaxial section, one yof said coupling elements .being electrically `connected to said input connection and the other of said .coupling elements .being .electrically connected to said output connection.

4. In .a .piston-type variable Iattenuator of .the lcharacter described, input and .output Vconnections lrigidly fixed with respect 4to each other, .input and output tubular coaxial sections, an intermediate .tubular coaxial section telescopically :disposed between each of said first named fcoaxial sections and .axially movable with respect to each fof lthe latter, a iirst .electromagnetic energy coupling .element lying within :and in xed relationship :with .respect to one of said first named coaxial sections, anda second :electromagnetic energy coupling element lying within and in fixed relationship with respect .to said intermediate .coaxial section, one vof said coupling elements beingelectrically connected to .said `input connection and lthe other of said coupling elements being electrically connected to ysaid output connection, a tubular ,housing enclosing `said :three coaxial sections and .provided with a longitudinally extending slot, a irack within said slot iixed to said intermediate coaxial section, .fand a `pinion yin `mesh with said rack 'for .moving .said intermediate coaxial section,

References Cited in the tile of this patent UNTE'D 'STATES `PATENTS 2,508,426 Sensiper May 23, 1950 2,510,614 Weber June 6, 1-950 2,514,344 .Slaymaker July 4, .1950 2,514,544 .Hansen July .11, 1950 2,516,060 Levenson July 1.8, 19.50 .2,525,554 1Latimer Oct. 1.0, 1950 2,557,110 Jaynes June 19, 1951 FOREIGN PATENTS 897,704 France Mar. 29, 1945 

